1. Field of the Invention
This invention concerns improvements in coal extraction more especially it concerns the treatment of solvent for coal extraction.
2. Description of the Related Art
It is known, from our UK Patent Application No GB 2156841A, that it is desirable for efficient solvent extraction of coal to reduce the saturation of the recycled solvent, arising from over hydrogenation.
It is well-known that coals can be dissolved in oil-type solvents at high temperatures, and that improved yields can be obtained by the presence of hydrogen, under high to low pressures of hydrogen or through the use of so-called hydrogen donor solvents. Catalysts may be present.
Various processes have been suggested for the liquefaction of coal using a so-called hydrogen donor solvent. A hydrogen donor solvent can be defined as an oil or fraction of an oil boiling in the range 200.degree.-500.degree. C., which is essentially hydroaromatic in composition and can donate its chemically bound hydrogen to the depolymerizing coal at high temperature, stabilizing the coal extract produced by addiing the hydrogen to the coal radicals, and thus preventing the radicals from forming coke.
A typical process, by way of example, for the liquefaction and destructive hydrogenation of coal, would consist of contacting crushed coal with a hydrogen donor solvent at high temperature in a first reactor to dissolve the coal, followed by filtration to remove ash and undissolved coal, in a second reactor the coal extract, together with the solvent or a fraction of the solvent, is contacted with a catalyst in a fixed bed together with hydrogen at high pressure and temperature. The coal extract is converted to distillable oils and the solvent is replenished with hydrogen donors. After fractionation of the products, the light oils can be further upgraded to gasoline, diesel and aviation fuels, and the hydrogen donor solvent can be recycled to the first reactor to dissolve more coal. Hence, the process can be made continuous and independent of external sources of solvent.
A problem with the above-described process is the high pressure of hydrogen required to convert adequately the coal extract in the second reaction and to prevent coking at the high reactor temperature. The high hydrogen pressure tends to give a recycle solvent which becomes saturated with hydrogen on multiple passes through the second reactor. Compounds such as alkyl decalins, perhydrophenanthrene and perhydropyrene are formed on repeated cycles. These compounds are paraffinic in nature and can cause precipitation of the dissolved coal extract leading to precipitates blocking process lines.
Furthermore, the saturated compounds are poor hydrogen donors relative to the hydroaromatic compounds, which leads to lower extraction yields.
The saturates in the recycle solvent could, in theory, be removed by a number of methods, for instance liquid/liquid extraction or reaction of the saturates with elemental suphur or selenium. Liquid/liquid extraction is inconvenient and leads to a loss of solvent from the process. Reaction with sulphur requires large quantities of the element and produces a large quantity of hydrogen sulphide which is undesirable.
Another approach to the problem of over-hydrogenation of the recycle solvent is described in British Patent Application Number 82/03640. A solvent consisting of aromatic polycyclic hydrocarbons of three and/or four ring molecules and at least 25% of saturated naphthenes boiling in the range 180.degree.-300.degree. C. is employed. The aromatic portion of the solvent in the process is removed by distillation after extraction of the coal so that it does not pass through the hydrocraker and subsequently saturates are not allowed to increase on repeated recycle.
According to GB-A-2156841 coal liquefaction processes employing hydrogen donor solvents are improved by dehydrogenating the saturates contained in the recycle solvent or a fraction of it to hydroaromatics thus removing the chemical entites which cause precipitation of coal extract without losing solvent balance in the overall hydroliquefaction process. Dehydrogenation of the saturates to hydroaromatics enables the process to operate without the problem of precipitates in process lines and advantageously enables the hydrocracker to operate at high pressures, for instance 200 atmospheres, which are necessary to achieve high conversion of coal extract in the presence of the hydrogen donor solvent.
GB-A-2156841 therefore provides a method of coal liquefaction in which coal is extracted using a liquid hydrogen donor solvent at elevated temperature and pressure at least a fraction of the extract and at least a fraction of the solvent are hydrogenated together or separately and at least a portion of the hydrogenated solvent is recycled to the extraction, characterised in that part at least of the solvent is catalytically dehydrogenated to reduce the quantity of cyclic saturates. The part of the solvent which is catalytically dehydrogenated may be taken from any point of the cyclic liquefaction process, and the dehydrogenation may be carried out continuously or intermittently.
The part of the solvent which is dehydrogenated would contain between 5 and 95% of weight of saturates, but preferably contained 10 to 20% of saturates, and it might contain 95 to 5% by weight of aromatics, but the aromatic content was preferably rather low, for example 5 to 25%. It was preferred to dehydrogenate saturates to hydroamatics, since it was thought that aromatics might inhibit the catalyst.
The catalytic dehyrogenation could be carried out in a method analogous to the reforming of naphtha in petroleum oil refineries. It was not the practice, however, to reform factions having the chemical composition of the hydrogenated solvent, nor did naphtha have similar cut points. The catalyst had to be capable of converting cyclic saturates to hydroaromatics, and would also thus convert hydroaromatics to aromatics although this latter reaction was less desirable. A careful selection by testing was, however, necessary since nickel/molybdenum or alumina converted hydroaromatics to aromatics but cyclic saturates were unconverted. Preferred catalysts included platinum and/or palladium on alumina, silica or active carbon at a loading of 0.1 to 10%, preferably 0.2 to 1% by weight; these readily promoted the dehydrogenation of saturates such as decalins to tetralins, perhydrophenanthrenes to octa-and tetra-hydrophenanthrenes and perhydrophyrenes to hexa- and di-hydropyrenes. Another preferred catalyst was chromia on alumina. The catalyst could be used in a fixed of fluidised bed reactor.
The catalytic dehydrogenation was suitably carried out at pressures of from 1 to 40 bar, preferably 15 to 25 bar over a platinum catalyst and preferably 1 to 5 bar over a chormia catalyst and suitable temperatures are from 400.degree. to 550.degree. C., preferably 460.degree. to 480.degree. C. Flowrates of hydrogenated solvent, measured as liquid hourly space velocity, were suitably 0.2 to h-Oh-1, but tests over a platinum catalyst indicated that flow rates of 0.5 to 1.0 were most preferred. Hydrogen might require to be fed to the process in order to achieve a high hydrogen partial pressure. Hydrogen to solvent molar ratios (H2:HC) were suitably 3 to 20, but are preferably 5 to 10.
Dehydrogenation catalyst was susceptible to poisoning, especially by sulphur, and it was desirable to ensure that the solvent steam being treated was low in catalyst poisons. If the solvent stream to be treated was not sufficiently free from catalyst poisons, then it was preferred to desulphurise the stream, for example by hydrogenating over a Ni/Mo or Co/Mo catalyst; this way also effective to reduce the nitrogen content of the stream.